LOS ANGELES (AP), March 16 - Four more tubes that carry radioactive water at a California nuclear power plant failed pressure tests, prompting new safety concerns, officials disclosed Friday.

The four tubes in a massive steam generator failed Thursday in the Unit 3 reactor at the San Onofre coastal plant in San Diego County, Southern California Edison said. Three other tubes failed earlier tests, the company said Wednesday, bringing the total to seven.

The utility shut down Unit 3 and began testing samples from thousands of tubes on Jan. 31 after a leak was found. Traces of radiation escaped during the leak, but officials said there was no danger to workers or neighbors.

The Nuclear Regulatory Commission said Thursday it was sending a special team of inspectors to try to determine why the metal tubes, which were installed in Unit 3 in 2010, have become frail enough to pose a risk of leaks.

Investigators have been looking into excessive wear on tubes in Unit 3 and its twin, Unit 2, which has been off line for maintenance and refueling. In a $670 million overhaul, two huge steam generators, each containing 9,700 tubes, were replaced in Unit 2 in fall 2009 and a year later in Unit 3.

Nineteen percent of all power used by SCE customers comes from nuclear generation.

A spokeswoman for the agency that operates the state's wholesale power system, the California Independent System Operator, said the San Diego and Los Angeles areas could see rotating power outages this summer if both reactors remain off line. The agency is taking steps to prevent those shortages.

Inside a steam generator, hot, pressurized water flowing through bundles of tubes heats non-radioactive water surrounding them. The resulting steam is used to turn turbines to make electricity.

The tubes are one of the primary barriers between the radioactive and non-radioactive sides of the plant, according to the NRC. If a tube breaks, there is the potential that radioactivity from the system that pumps water through the reactor could escape into the atmosphere.

Serious leaks also can drain cooling water from a reactor, said David Lochbaum, director of the nuclear safety project for Union of Concerned Scientists.

The plant is owned by Edison, San Diego Gas & Electric and the city of Riverside. Southern California Edison serves nearly 14 million residents with electricity in Central and Southern California.